In a base station used for mobile wireless communication, comparatively high power transmission signals and comparatively weak reception signals pass through the same path from a front end to an antenna of a base station. If a component such as cable and connector in such a path exhibits nonlinear behavior, intermodulation distortion (IMD) due to higher order (e.g., third order) nonlinearities can adversely impact the received signal and degrade performance. As can be appreciated, in certain applications, measurement of IMD can be useful to improve performance in wireless communication systems.
A known measurement apparatus determines a length of a cable from a test port to a passive intermodulation (PIM) source. In known the apparatus, an oscillator generates a sawtooth wave, and another oscillator generates a frequency modulated (FM) wave at a first frequency F1 (where sweep rate is ΔF/T). Another oscillator generates a wave at a second frequency F2, and a combiner mixes the waves of both frequencies to supply a test port. An output signal synthesized at the combiner is transmitted to the test port through an inner line and is further transmitted to the measured cable that is connected to the test port. An intermediate connection portion or a termination portion of the cable may be an interface at which a shape of the cable discontinuously changes or an interface at which dissimilar metals are in contact, and nonlinear distortion is often generated at the intermediate connection portion. When nonlinear distortion is generated at such an interface, the location of the interface is determined and is identified as a PIM generation source.
A third IMD signal is generated at the PIM generation source and has a frequency (2F1−F2) or (2F2−F1) that is transmitted through the cable in a return direction and arrives at the test port. If a pass frequency of a bandpass filter is set to (2F1−F2), a signal of the frequency (2F1−F2) passes through the bandpass filter and reaches a first input terminal of at a frequency mixer.
Meanwhile, based on the signal of the frequency F1 of the first oscillator, a higher harmonic wave 2F1 of twice the frequency is formed at a frequency doubler. The higher harmonic wave is mixed with the signal of the frequency F2 generated by the second oscillator at the frequency mixer, and the resulting signal of 2F1-F2 passes through the bandpass filter and is received at the frequency mixer 48.
In the known apparatus it is presumed that the two bandpass filters have same frequency and phase characteristics, group delay of the cable is independent of frequency, and a time lag of the other components is zero.
In the known apparatus, signals are transmitted at frequencies F1, F2 and are converted to a signal of frequency (2F1−F2) at the PIM generation source that returns to the port P1. The time delay becomes 2Td since the group delay Td of the cable is presumed to be constant in different frequencies. As a result, a distorted signal (Vu) generated at the PIM generation source arrives at an input of the frequency mixer with a delay of 2Td compared with a reference signal (Vr). Thus, the frequency of the signal Vu differs to the frequency of the reference signal Vr by 2Td·ΔF/T.
The shift of frequencies enables a time delay in the cable to be measured, and a physical length of L can be calculated if a wavelength shortening of the cable is known.
With the known measurement apparatus, the two bandpass filters used have a sharp bandpass characteristic passing (2F1−F2) component with enough attenuation of F1 and F2 components so that the receiver is not saturated, and so the frequency and phase characteristic of the bandpass filters should be the same. If there is a difference in the amplitude and phase characteristics of the two bandpass filters, a measurement error will be included in the measured value caused by the difference of the characteristics. However, bandpass filters having the sharp bandpass characteristic are comparatively expensive. Furthermore, two bandpass filters having sufficiently precise frequency and phase characteristics is also difficult.
In the known measurement apparatus, the reference signal Vr is generated with the aid of the frequency mixer, the frequency doubler and the bandpass filter. In the frequency mixer, a converted output signal appears not only at an output terminal but also at an input terminal. Therefore, to realize a dynamic range of more than 160 dBc in the PIM measurement, the converted signal at the input terminal of the frequency mixer should be prevented from reflecting back to the signal sources. In order to prevent this reflection, buffer circuits are required between the signal source and the frequency doubler, and between the frequency source and the frequency mixer. As can be appreciated preventing this reflection requires a rather complicated circuit configuration.
What is needed, therefore is a PIM measurement apparatus that overcomes at least the shortcomings of the known apparatus described above.